1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic process, an electrostatic recording process, or the like and has developing means which performs a development operation with a contact development by a developer carrying member.
2. Related Background Art
Conventionally, in an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus provided with a function for forming an image on a recording medium, an electrostatic latent image formed on an image bearing member such as a photosensitive drum is visualized as a developer image (toner image) with developing means using a developer (toner).
As such developing means, for example, a dry mono-component contact development has been proposed and put to practical use. In this system, an electrostatic latent image is often developed by bringing a rotating image bearing member (photosensitive drum) and a developer carrying member such as a developing roller bearing a developer, which is also rotating, into pressed contact or contact with each other with an appropriate relative peripheral speed difference. In addition, in this system, there are many advantages such as a magnetic material is unnecessary, an apparatus can be easily simplified and miniaturized, and the developing means employing the system can be applied to a full color image forming apparatus because a mono-component developer containing a nonmagnetic toner is used.
In recent years, in the midst of diversification of demands, a request for colorization of an image to be outputted by a printer used, for example, in an office or the like has been increasing.
In an attempt to respond to such a request, color image forming apparatuses according to several techniques have been proposed. As an example, a color image forming apparatus of an intermediate transfer member system has been proposed. The color image forming apparatus aims to obtain a color image without color drift by providing, other than a first image bearing member such as a photosensitive drum on which surface a toner image is formed, an intermediate transfer member serving as a second image bearing member to which plural colors of toner images being superimposed on one another are to be primarily transferred from the first image bearing member, and secondarily transferring a complex toner image of the plural colors formed on the intermediate transfer member collectively to a transfer material.
FIG. 12 shows a schematic diagram thereof. In the figure, first, a photosensitive drum 101, which is driven to rotate in a direction of an arrow at a predetermined process speed, is uniformly charged by a charging roller 102 on its surface. Next, the photosensitive drum 101 is subjected to scanning exposure by a laser beam 103 which is ON/OFF controlled according to image information, thus forming an electrostatic latent image on the photosensitive drum 101.
This electrostatic latent image is developed by developing means containing a developer. In this example, the electrostatic latent image is developed and visualized by a rotary developing apparatus 104 which is capable of switching each developing means through rotation.
In this rotary developing apparatus 104, the four developing means each have a developing roller 111 serving as a developer carrying member. The rotary developing apparatus 104 is constituted by integrally forming the four developing means, namely, a first developing device 104a containing a yellow toner as a developer (toner) of a first color, a second developing device 104b containing a magenta toner as a developer (toner) of a second color, a third developing device 104c containing a cyan toner as a developer (toner) of a third color, and a fourth developing device 104d containing a black toner as a developer (toner) of a fourth color. First, the electrostatic latent image is developed and visualized by the first developing device 104a containing the yellow toner as the toner of the first color. Note that, these toners are mono-component nonmagnetic toner, and a normal polarity thereof is negative.
A visualized first toner image is electrostatically transferred (primarily transferred) onto a surface of an intermediate transfer belt 105, which serves as an intermediate transfer member to be driven to rotate in the direction of the arrow, in a first transfer region 106 which is opposed to the intermediate transfer belt 105. Note that primary transfer remaining toner, a slight amount of which remains on the surface of the photosensitive drum 101 having finished primary transfer, is removed by a cleaning device 107. This cleaning device 107 has a cleaning blade 107a including an elastic member which abuts against the surface of the photosensitive drum 101 in a so-called counter direction.
Subsequently, toner images are sequentially superimposed and transferred onto the intermediate transfer belt 105 by repeating the above-mentioned process three times using the second to fourth developing devices 104b to 104d. That is, a second toner image developed with the magenta toner, a third toner image developed with the cyan toner, and a fourth toner image developed with the black toner are sequentially transferred to and stacked on the surface of the intermediate transfer belt 105.
Thereafter, a secondary transfer roller 108, which has been in a separated state with respect to the surface of the intermediate transfer belt 105, is brought into pressed contact with the surface of the intermediate transfer belt 105 and is driven to rotate. The toner images formed on the surface of the intermediate transfer belt 105 are collectively transferred (secondarily transferred) onto a surface of a transfer material P which is conveyed at predetermined timing to a second transfer region 109. This transfer material P is conveyed to a fixing device 110 to have the toner images fixed as a permanent image, and then discharged to the outside of the apparatus.
Here, the respective developing devices 104a, 104b, 104c, and 104d are in the form of a cartridge and have: the developing roller 111 serving as the developer carrying member; a developing blade 112 serving as a developer regulating member which abuts against the developing roller 111 to regulate an amount of toner on the developing roller 111; a supply roller 113 which abuts against the developing roller 111 to supply a mono-component nonmagnetic toner to the developing roller 111; and an agitating member 114 which carries a toner to the vicinity of the supply roller 113. The developing devices 104a, 104b, 104c, and 104d are detachably attachable to the developing apparatus 104 in the image forming apparatus.
Note that the developing roller 111 rotates while abutting against the photosensitive drum 101 to thereby perform a development operation of carrying a toner contained in the developing means and feeding the toner to an electrostatic latent image portion on the surface of the photosensitive drum 101. Therefore, the developing roller 111 preferably has at least an elastic body in order to be compatible with a so-called contact development. Then, a predetermined DC bias is supplied to the developing roller 111 from a development bias power supply (not shown) in order to transfer the toner from the developing roller 111 to the surface of the photosensitive drum 101.
As the developer carrying member, a developing roller having elasticity and electrical conductivity is often used. That is, since development is performed by bringing the developing roller into pressed contact or contact with an image bearing member, in particular, in the case in which the image bearing member is a rigid body, the developing roller is constituted by an elastic body in order to prevent damage to the image bearing member.
In addition, the developing blade 112 is brought into abutment against the surface of the developing roller 111 with a light pressure utilizing spring elasticity of a thin metal plate.
However, in such an image forming apparatus using a mono-component nonmagnetic toner, deficiencies as described below may occur.
In order to obtain a satisfactory image quality, a toner charging amount is required to be appropriately large. This amount depends upon a charging amount the toner itself and a charging amount which is obtained by triboelectrification due to rubbing in an abutment nip portion between a developing roller and a developing blade to which the toner is carried in accordance with the rotation of the developing roller.
Then, for example, in the case in which image formation is performed after the toner is left as it is for a long time, the charging amount of the toner itself in the developing means is decayed. In a very early stage immediately after starting up the apparatus, in particular, in the case in which a mono-component nonmagnetic toner not using a carrier is used, image formation is started in a state in which a sufficient charge is not given to the toner and a sufficient charging amount is not obtained, or the charging amount cannot be maintained uniformly. Thus, an image failure such as a low image density or a non-uniform image density may occur.
In particular, in a so-called contact development which is adopted in the image forming apparatus of FIG. 12, fluctuation in charging of a toner, which occurs on the developing roller 111, easily becomes evident as unevenness of an image.
In addition, this phenomenon tends to occur conspicuously in a situation in which it is hard to obtain a sufficient toner charging amount, for example, in the case in which an amount of toner remaining in the developing means is large, or in a situation in which the image forming apparatus is installed in an environment of high temperature and high humidity.
Further, in such an image forming apparatus using a mono-component nonmagnetic toner, in the case in which developing means being in an unused state is used for the first time to perform image formation, deficiencies as described below may occur.
In general, the developing means in an unused state is provided with a seal member between a toner container portion, in which a toner is contained, and a development portion in which members such as a developing roller and a developing blade are disposed. The developing means is brought into a usable state upon removal of this seal member by a user. However, in an early stage of use, it is hard to coat the toner on the surface of the developing roller uniformly and, if image formation is performed in this state, image failure such as a non-uniform image density may occur.
In addition, in order to obtain a satisfactory image quality, a toner charging amount is required to be appropriately large. This amount depends upon a charging amount of the toner itself and a charging amount which is obtained by triboelectrification due to rubbing in an abutment nip portion between a developing roller and a developing blade to which the toner is carried in accordance with the rotation of the developing roller.
However, in the case in which image formation is performed using developing means in an unused state., the charging amount of the toner itself in the developing means is decayed. In a very early stage immediately after starting to use the developing means, image formation is started in a state in which a sufficient charge is not given to the toner and a sufficient charging amount is not obtained, or the charging amount cannot be maintained uniformly. Thus, an image failure such as a low image density or a non-uniform image density may occur. In particular, in a so-called contact development which is adopted in the image forming apparatus of FIG. 12, fluctuation in charging of a toner on the developing roller 111 easily becomes evident as unevenness of an image.
This phenomenon occurs very easily in developing means which has a less toner carrying force caused by a magnetic force compared with a development using a developing sleeve, which includes a magnet in its inside, and a magnetic toner. In particular, the phenomenon is more conspicuous in a mono-component nonmagnetic development using a mono-component nonmagnetic toner which has a substantially spherical shape.
In addition, this phenomenon tends to occur more conspicuously in a situation in which it is hard to obtain a sufficient toner charging amount, for example, in a situation in which the image forming apparatus is installed in an environment of high temperature and high humidity.